Liquid crystals are used primarily as dielectrics in indicating devices, since the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well-known to a person skilled in the art and can be based on various effects. Such devices are, for example, cells having dynamic scattering, DAP cells (deformation of aligned phases), guest/host cells, TN cells having a twisted nematic structure, STN cells ("super twisted nematic"), SBE cells ("super birefringence effect") and OMI cells ("optical mode interference"). For displays having a high density of information, actively controlled cells, for example TFT cells ("thin film transistor"), have recently become especially important in addition to the passively controlled, multiplexed cells. The most common indicating devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid crystal materials must have a good chemical, photochemical and thermal stability and a good stability towards electric fields. Further, they should have not only a suitable mesophase over a range which is as broad as possible (for example, a nematic or cholesteric phase for the aforementioned cells), but also a sufficiently low viscosity and in the cells should permit short response times, low threshold potentials and a high contrast. Further properties such as the electrical conductivity, the dielectric anisotropy and the optical anisotropy must fulfil different requirements depending on the field of application and type of cell. For example, materials for cells having a twisted nematic structure should have a positive dielectric anisotropy which is as high as possible and simultaneously should have a conductivity which is as low as possible. This latter property is especially important primarily for TFT cells. Unfortunately, however, liquid crystal materials having a high dielectric anisotropy lead mainly to an increased conductivity in mixtures as a result of their improved capability of dissolving ionic impurities. Accordingly, a need exists for liquid crystal materials which have a dielectric anisotropy which is as high as possible with a conductivity which is as low as possible.